Pressure and heart movement sensor for heart stimulators

Abstract: A pressure and heart movement sensor for a heart stimulator, comprising an electrode system for implantation with an electrode cable containing at least two electrical conductors (2, 4) leading to electrode poles (6, 8). A measurement device (12, 14, 16, 18, 20) is arranged to measure an electric quantity (C₁) between the electrical conductors, which quantity varies according to the pressure applied to the electrode cable. Alternately, the electrode cable contains at least one electrical conductor, and the measurement device is arranged to measure the impedance (C₂, C₃) between the conductor and ambient medium and deliver an output signal, which varies according to the impedance measured, when the electrode cable is subjected to pressure.

Description: This invention relates to a pressure and heart movement sensor for a heart stimulator, comprising an electrode system for implantation with an electrode cable containing at least one conductor leading to one electrode pole, or alternately at least two contuctors leading to electrode poles. For arrhythmia-treating implants, such as heart stimulators, pressure sensors for controlling the stimulator are ideal sensors in many respects. With a pressure sensor, hemodynamics can be measured in both tachyarrhythmias and bradyarrhythmias. Heart contractions can be sensed and used for autocapture and atrial synchronization. Furthermore, the time derivative dP/dt is a good rate-response parameter. For the applications in question, prior art pressure sensors normally contain a piezoelectric or piezoresistive pressure element. A pacemaker controlled with this type of sensor is previously known from e.g. US-A-4,140,132. One disadvantage of this type of sensor is that the sensor requires additional electrical lines, normally two. The electrode cable must therefore contain relatively many conductors. Another disadvantage of this design is that special impedance demands must often be made on the pressure element in order e.g. to eliminate the problem of leakage resistances, making it necessary to place electric components out near the sensor itself. Another disadvantage is the fact that the piezoelectric ceramic material itself is hard and brittle. Capacitive accelerometers are also previously known, cf. DE-A1-3 223 987 in which an inertial mass forms one of the capacitor plates so the capacitance changes when the plate is subjected to acceleration. However, this technic cannot be used for implantable pressure and heart movement sensors of the kind involved here. Such a sensor would also require additional conductors. The existence of a capacitive coupling between electrical conductors and between a conductor and ambient medium is well-known, cf.e.g. Tietze and Schenk, Halbleiter-Schaltungstechnik, Springer-Verlag, Berlin Heidelberg New York 1978, 4th edition, page 647. The magnitude of the capacitive coupling depends a.o. on the dielectric constant of the materials between the conductors and the distance between them. Such a capacitive coupling is naturally found even between the conductors in a bipolar pacemaker electrode system and between the conductors and ambient electrolyte, i.e. blood. A capacitance of about 100 pF has been measured between the two conductors in a bipolar pacemaker electrode system in dry conditions. The two conductors here must of course not terminate in electrical poles in direct contact with one another via the electrolyte or body fluid, since this would result in short-circuiting of the capacitance. The resistance between the conductors must be sufficiently large. The shape of a bipolar electrode system implanted in a patient changes when the patient moves, thereby changing the capacitance between electrode conductors and between the conductors and ambient electrolyte. The patient's heart movements will therefore be expressed as changes in capacitance. If a medium, whose resistivity or piezoelectric voltage changes when the conductors are moved or deformed by the pressure of blood or heart movements, is arranged between the conductors in a bipolar electrode system, changes in pressure and heart movements will appear as changes in resistance or voltage between the conductors. The purpose of the present invention is to achieve, by utilization of these effects, a new type of pressure and heart movement sensor in which the above-described disadvantages of previously known sensors for controlling heart stimulators are eliminated. This purpose is achieved with a pressure and heart movement sensor of the type defined in the introduction with the characterizing features set forth in claims 1 or 22. Thus, the pressure and heart movement sensor according to the invention is based on the circumstance that pressure in the heart increases every time the heart beats, this increasing pressure compressing the electrode system so the medium between the conductors of the electrode system is compressed with attendant changes in the medium's electrical properties. Furthermore, the electrode and its conductor may move every time the heart beats. These changes in shape or deformations of the electrode system cause changes in the distance between the electrode conductors in e.g. a bipolar electrode system or between an electrode conductor and ambient electrolyte. This change in distance, as well as the above mentioned changes in the electrical properties of the intermediate medium, result in a change of the impedance between electrode conductors and ambient electrolyte or in a change in impedance or piezoelectric voltage between two electrode conductors, which electrical quantities can be measured. In this very simple manner, the patient's heart movements and pressure changes can be detected with the ordinary heart stimulator electrode. According to one advantageous embodiment, the sensor medium is arranged to form at least a part of one of the conductors, and the measurement device is connected to two points on this conductor, on either side of an area of the sensor medium, in order to measure changes in an electrical quantity in this area when the sensor medium is subjected to pressure. This is an advantageous, practical realization of the invention. According to another advantageous embodiment of the sensor according to the invention, the sensor medium comprises an insulating, elastic substance, such as silicone rubber, containing granules of conductive material, such as granules of coal. When the sensor medium is subjected to pressure, the resistance between the conductors changes because of changes in inter-granular contacts. When the conductors are compressed more contacts are formed between the coal granules and the resistance drops. According to advantageous refinements of the sensor according to the invention, the silicone rubber filling in the preceding embodiment is replaced with a carbon fiber mat, a conductive polymer or a sodium chloride solution, thereby improving conductivity. In other advantageous embodiments of the sensor according to the invention, the sensor medium contains non-conductive granulate, such as glass or ceramic material, and a conductive polymer. When the electrode cable is compressed, thereby reducing the distance between the conductors, the density of insulating material increases, thus leading to a change in the resistance between the conductors. In an additional advantageous embodiment of the sensor according to the invention, the sensor medium comprises a piezoelectric material, preferably in powder form, and silicone rubber. This results in a piezoelectric based pressure and heart movement sensor, with no loss of electrode cable flexibility and no hard, brittle piezoelectric elements, with which the piezoelectric voltage can be measured between existing electrode conductors. According to yet another advantageous embodiment of the sensor according to the invention, the conductors are formed of two electrical conductors, diametrically opposite one another, extending longitudinally inside a flexible hose. This thereby increases the sensor sensitivity. In a bipolar electrode system with an electrode tip connected to a helically wound inner conductor and a coaxial ring connected to a helically wound outer conductor, according to an advantageous embodiment of the sensor according to the invention, the sensor medium is arranged between the inner and outer helices and the impedance between the ring and tip is great enough to prevent the formation of an electrical connection between the electrode tip and the ring when the desired electrical quantity is measured between the conductors. In the case of a bipolar pacemaker electrode system with the conductors coiled in two helices, one inside the other, the impedance between the two helices or between the outer helix and the surroundings, or both impedances, can be used for detecting heart movements. When the impedance between electrode conductor and the surroundings is utilized, the enclosure of the heart stimulator serves as a connection for the surroundings. The capacitance can obviously be used as quantity of measure instead of the impedance. According to another advantageous embodiment of the sensor according to the invention, the conductors (s) of the electrode cable are enclosed in silicone which swells when penetrated by fluid so its dielectric constant changes, leading, in turn, to an increase in the capacitance to be measured. This is an advantageous effect, since it facilitates measurement of the capacitance, and a more reliable sensor is attained. According to further advantageous embodiments of the sensor according to the invention, the measurement device comprises a measurement oscillator, devised so its frequency changes according to the electrical quantity measured. A differentiator is suitably arranged to determine the difference between the frequency dependent on the electrical quantity from the measurement oscillator and a reference frequency supplied by a reference oscillator. To the differentiator's output terminal a frequency/voltage converter is advantageously connected which delivers an output voltage signal, representing the frequency difference and, thus, the change in the electrical quantity. As exemplifications, selected embodiments of the sensor according to the invention will now be described more in detail, referring to the attached drawings in which FIG. 1 shows a part of the electrode cable in a bipolarelectrode system with two conductors arranged in a silicone rubber flexible tube; FIG.2 shows a version of the sensor according to the invention in which the sensor medium constitutes a part of a conductor; FIG. 3 illustrates a bipolar electrode system with two helically coiled conductors, one inside the other; FIG. 4 schematically depicts the capacitance conditions in the electrode system in FIG. 3; FIG. 5 shows a coaxial version of the sensor according to the invention; FIG. 6 depicts a diagram of the measurement device for converting changes in the measured electrical quantity in an output voltage signal; FIG. 7 shows an example of the output signal from the measurement device in FIG. 6 and FIG. 8 shows how an oscillator with a capacitance-dependent frequency can be realized in the measurement device in FIG. 6. FIG. 1 shows a part of the electrode cable in a bipolar electrode system with two conductors 1 arranged inside a silicone rubber flexible tube 3. The conductors 1 are arranged diametrically opposite to one another at the inner side of the flexible tube 3 and extend in the latter's longitudinal direction. The internal diameter of the flexible tube 3 is typically 2 mm, and the wall thickness is a few tenths of a mm. The flexible tube 3 is filled with a mixture 5 of coal granules, or coal granulate, and silicone rubber which fills the entire flexible tube 3. The coal granulate content should exceed 40%. The resistance measured between the conductors 1 changes when the flexible tube 3 is subjected to pressure variations caused by heart movements or variations in blood pressure. When the flexible tube 3 is compressed such that the distance between the conductors 1 decreases, the resistance measured between the conductors 1 drops, because of improved contact between the electrically conductive coal granules. Instead of being filled with the above mixture 5 of coal granules and silicone rubber, the flexible tube 3 can be filled with a mixture 5 of coal granules and a conductive polymer or a mixture 5 of coal granules and sodium chloride solution, thereby improving the interconduction. Improved conduction between the conductors 1 is also achieved if the flexible tube 3 is filled with a mixture 5 of carbon fiber mat and silicone rubber. The mixture 5 in the flexible tube 3 can also consist of a conductive polymer and non-conductive granulate, such as glass or ceramic material. When the flexible tube 3 is compressed in this instance so the distance between the conductors 1 decreases, the resistance changes between the conductors, since the density of granules of insulating material increases. The flexible tube 3 can also be filled with a mixture 5 of a piezoresistive powder and silicone rubber or a piezoelectric powder and silicone rubber. In the latter instance, a piezoelectric voltage, which varies with the electrode cable's deformation, can be measured between the conductors 1. The piezoelectric powder is made by crushing a piezoelectric ceramic material. Since the piezoelectric crystals are hard and brittle, a solution as above according to the invention has strength advantages compared to the use of conventional piezoelectric elements. In addition, the electric quantity of interest is measured between existing electrode conductors with no need for any additional conductors. In FIG. 2 is schematically shown an embodiment in which the sensor medium 22 is enclosed in a cavity inside an elastic tube 24, made from insulating material, between two conductive plates 26, 28 to which conductors 30, 32 are connected. The measurement device is connected to the conductors 30, 32 to measure e.g. the impedance between the plates 26, 28. In addition to the abovementioned types of sensor media, whose electrical properties change when the media are compressed, conductive media, such as electrolytes or conductive gels, can also be used as sensor medium 22 in this instance. Here, the sensor medium 22 will constitute a part of the conductor itself 30, 32, and measured changes in impedance, when the part of the flexible tube containing the sensor medium 22 is compressed, are caused by geometric changes in the conductive path. Thus, compression of the flexible tube 24 can result in a reduction in the cross-sectional area of the sensor medium 22 in a given area, thereby narrowing the current path. Instead of using a fluid or gel sensor medium, enclosed in an outer sleeve, the actual sensor part can naturally be devised in the form of a solid body made from an appropriate sensor medium and arranged between the conductors 30, 32. In FIG. 3 is shown a bipolar electrode system with two helically coiled conductors 2, 4, one inside the other. The inner conductor 4 is connected to a tip or stimulation electrode 6, and the outer conductor 2 is connected to an indifferent ring electrode 8. The conductors 2, 4 are enclosed in an insulation 10. The insulation 10 can consist of a silicone which swells when penetrated by fluid, leading to a change in the dielectric constant and an increase in capacitance. The sensor sensitivity also increases as a result. Between the conductors 2, 4, and between the conductors 2, 4 and the ambient medium, there are capacitive couplings, schematically represented in FIG. 4 by the capacitances C 1 and C 2 , C 3 respectively. In dry condition, the capacitance C 1 has been measured to about 100 pF, provided that the electrode poles 6, 8 are not in contact with one another via electrolyte or body fluid. As shown in FIG. 3, the ring electrode 8 is therefore fully encapsulated against the stimulation electrode 6. The capacitance C 1 between the conductors 2, 4 changes with the distance between the conductors 2, 4. Movement and changes in pressure deform the electrode conductors 2, 4, leading to changes in both the capacitance C 1 and the capacitances C 2 and C 3 . In this manner, heart movements will cause changes in the capacitance. Heart movements can thus be sensed by continuous monitoring of the capacitance. In practice, the capacitance C 1 between the conductors 2, 4 or the capacitance C 2 between the outer conductor 2 and ambient medium, or both these capacitances, can be used. In FIG. 5 is shown a coaxial version with an inner conductor 34 in the form of a metal helix, surrounded by a layer 36 of sensor medium. A layer 38 of conductive material, to which a second conductor 40 is connected, is arranged on the exterior of the sensor medium 36. A representative electrical quantity, varying with changes in the pressure on the device, can be measured between the conductors 34, 40. In FIG. 6 is shown a pressure and heart movement sensor according to the invention which, thus, contains a measurement device with a measurement oscillator 14 the frequency f mät of which depends on changes in the electrical properties of a sensor medium and/or changes in the measured electrical quantity as a result of geometric changes caused by movement and deformation of the conductors. The measurement device further comprises a reference oscillator 16 with a fixed reference frequency f ref . The measurement oscillator's fundamental or centre frequency is appropriately chosen equal to the frequency of the reference oscillator, and the frequencies f mät and f ref are supplied to a differentiator 18 in which the difference between the two frequencies f mät and f ref is established. This differential frequency f diff is supplied to a frequency/voltage converter 20 which converts the differential frequency f diff in a corresponding output voltage signal U, see FIG. 6. In FIG. 7 is shown an example of the output voltage U versus time. A large output voltage U means a large frequency difference f diff which, in turn, corresponds to a large change in the representative electrical quantity caused by e.g. a major change in pressure or heart movement. In FIG. 8 is shown an example of a measurement oscillator 14 in FIG. 6 the frequency f(C) of which is a function of the capacitance C at the input. Thus, the capacitance C is connected to one input of an amplifier 12, the other input of which is connected to a fixed reference value, determined by the resistor R 1 . The amplifier 12 is further fed back with the resistors R 2 and R 3 as shown in FIG. 8. The oscillator's frequency f then becomes f = 1 2R 3 ·C·1n (1 + 2R 1 R 2 ) i.e. the frequency f becomes a function of the capacitance C. When the amplifier 12 is arranged so either the capacitance C 1 between the conductors 2,4 in the bipolar electrode system or the capacitance C 2 between the outer conductor 2 and the ambient electrolyte forms the capacitance C, an oscillator is obtained the output frequency f(C) of which is a function of the capacitances C 1 or C 2 and which, thus, changes when the shape of the electrode conductors changes, i.e. heart movements can be detected from the output frequency f(C). Above, the invention has been described in conjunction with a bipolar electrode system. However, the invention is also clearly applicable to an unipolar electrode system, the measurement oscillator being controlled by e.g. the capacitance between the conductor and the ambient electrolyte. The heart stimulator's capsule then forms the connection for the ambient medium.

Claims: A pressure and heart movement sensor for a heart stimulator, comprising an electrode system for implantation with an electrode cable containing at least two electrical conductors (1, 2, 4) leading to electrode poles (6, 8), characterized in that a measurement device (12, 14, 16, 18, 20) is arranged to measure an electric quantity between the electrical conductors (1, 2, 4) which quantity varies according to the pressure applied to the electrode cable. A sensor of claim 1, characterized in that a sensor medium is arranged between the conductors, the electrical properties of said medium changing when the medium is compressed when subjected to an external pressure, thereby causing changes in the electrical quantity measured. A sensor of claim 2, characterized in that the sensor medium is arranged to form at least a part of one of the conductors, and in that the measurement device is connected to two points on this conductor, on either side of an area of the sensor medium, in order to measure changes in the electrical quantity in this area, when the sensor medium is subjected to pressure. A sensor of claim 2 or 3, characterized in that the sensor medium comprises an insulating, elastic substance containing granules of conductive material. A sensor of claim 4, characterized in that the insulating, elastic substance is silicone rubber. A sensor of claim 4 or 5, characterized in that the granules of conductive material are granules of coal. A sensor of any of claims 2 - 4, characterized in that the sensor medium comprises a carbon fiber mat and silicone rubber. A sensor of any of claims 2 - 4, characterized in that the sensor medium comprises granules of coal and a conductive polymer. A sensor of claims 2 or 3, characterized in that the sensor medium comprises non-conductive granulate and a conductive polymer. A sensor of claim 9, characterized in that the non-conductive granulate consists of glass or ceramic material. A sensor of claims 2 or 3, characterized in that the sensor medium comprises a piezoresistive substance. A sensor of claims 2 or 3, characterized in that the sensor medium comprises a piezoelectric substance, preferably piezoelectric powder in silicone rubber, and in that the measurement device is devised to measure the piezoelectric voltage between the conductors. A sensor of claims 2 or 3, characterized in that the sensor medium comprises granules of coal and a conductive polymer (5). A sensor of claim 2 or 3, characterized in that the sensor medium comprises granules of coal and a sodium chloride solution (5). A sensor of claim 2 or 3, characterized in that the sensor medium is an electrolyte. A sensor of claim 2 or 3, characterized in that the sensor medium is an electrically conductive gel. A sensor of claims 1-16, characterized in that the conductors (1, 2, 4) move in relation to one another and/or are deformed when pressure is applied to the cable, and in that the electrical quantity measured varies according to the movement and/or deformation. A sensor of any of claims 1 - 17, characterized in that the conductors are separated from one another inside a flexible tube, made from a flexible, elastic and insulating material, such as silicone rubber and filled with sensor medium. A sensor of claim 18, characterized in that two diametrically opposed electrical conductors extend longitudinally along the inner side of the flexible tube. A sensor of any of claims 1 - 19, the electrode system being a bipolar electrode system with an electrode tip (6) connected to a helically wound inner conductor (4) and a coaxial ring (8) connected to a helically wound outer conductor (2), characterized in that a sensor medium is provided between the inner and outer helix, and in that the impedance between the ring (8) and tip (6) is sufficiently large. A sensor of any of claims 1-20, characterized in that the measurement device is devised to measure the resistance and/or the capacitance between the conductors. A pressure and heart movement sensor for a heart stimulator, comprising an electrode system for implantation with an electrode cable containing at least one electrical conductor (2, 4) leading to one electrode pole (6, 8), characterized in that a measurement device (12, 14, 16, 18, 20) is arranged to measure the impedance between the conductor (2, 4) and ambient medium and deliver an output signal, which varies according to the impedance measured, when the electrode cable is subjected to pressure. A sensor of claim 22, the electrode cable containing two helically coiled conductors (2, 4), one inside the other, characterized in that the measurement device (12, 14, 16, 18, 20) is arranged to measure the impedance between the outer helical conductor (2) and ambient medium. A sensor of claim 21 or 22, characterized in that the measurement device is arranged to measure the resistance and/or the capacitance. A sensor of any of claims 22-24, characterized in that the electrode cable conductor (2, 4) is enclosed in silicone (10) which swells when penetrated by fluid, such that its dielectric constant changes so the capacitance intended for measurement increases. A sensor of any of claims 1-25, characterized in that the measurement device comprises a measurement oscillator (14) so devised that its frequency (f mät ) changes according to the electrical quantity measured. A sensor of claim 26, characterized in that the measurement device comprises a reference oscillator (16) with a fixed reference frequency (f ref ) and a differentiator (18) which is arranged to determine the difference (f diff ) between the electrical quantity dependent frequency (f mät ) supplied by the measurement oscillator (14) and the reference frequency (f ref ). A sensor of claim 27, characterized in that the measurement device comprises a frequency/voltage converter (20), connected to the differentiator's (18) output, and delivering an output voltage (U) representing the frequency difference (f diff ) .

Inventors: HEINZE ROLAND, HOEGNELID KURT, HOLMSTROEM NILS, LJUNGSTROEM KARIN, NILSSON KENTH, WESTIN PIERRE, HEINZE, ROLAND, HOEGNELID, KURT, HOLMSTROEM, NILS, LJUNGSTROEM, KARIN, NILSSON, KENTH, WESTIN, PIERRE, Högnelid, Kurt, HOLMSTRÖM, Nils, Ljungström, Karin
Applicants: PACESETTER AB
Application year: 1994
Main classification: A61B5
IPC codes: A61N1, A61B5